The Pima Indians have an extremely high prevalence of obesity, with more than 70% of the adult population having a body mass index of over 27 kg/meters squared. Many animal models of spontaneous obesity, as well as obesity secondary to lesions of one hypothalamus, have been associated with abnormal sympathetic nervous system activity. Recent studies in obese Caucasians demonstrated an apparent abnormality of the sympathetic nervous system activity in obese subjects. This study was designed to study the relationship between obesity and autonomic nervous system activity in the Pima Indians as compared to an age, weight matched group of Caucasians. Preliminary analyses suggest that SNS activity contributes to stimulation of the metabolic rate in Caucasians but that this stimulation is absent in Pimas.